This invention relates to induction welding, and more particularly to inductors and a method for making inductors. Induction welding is used, among other things, for welding of the type of laminated material which is used in the packaging industry, that is to say, laminated material comprising layers of thermoplastics and aluminium foil. The technique is well-known and is based on the fact that a magnetic field around a conductor, through which passes an alternating current, is capable of inducing a current into an adjoining conducting material which, owing to the resistance of the material, causes a more or less pronounced heating of the material. In induction welding an induction loop or coil of the same configuration as that which the desired weld is to obtain is placed close to the laminate containing an aluminium foil, whereupon the laminate is pressed together with the material to which it is to be joined. The aluminium foil will now be heated in a pattern corresponding to the configuration of the coil and by appropriately selecting current, frequency and duration of the process, the material can be heated to the desired temperature, that is to say, a temperature which is sufficiently high for the layers of thermoplastic material adjoining one another to melt together.
In the packaging industry it is customary to use induction welding for the welding together of packages which are produced from laminated material comprising aluminium foil and thermoplastics. Induction welding is also used for the attachment by welding of sealing washers of aluminium foil and thermoplastics in caps, lids and the like.
The inductors used up to now for the welding of complicated welding patterns in the packaging industry consisted of a copper plate of 5-10 mm thickness, in which by spark-cutting, slits have been produced in such a pattern that the remaining parts of the plate form an induction coil of the desired configuration. The spark-cut plate is subsequently applied to a base of insulating material providing mechanical stiffness and embedded into plastic material in such a manner that the slits obtained by the spark-machining are wholly filled with the plastics serving as an insulating material. Then the side of the plate which is to serve as working surface is ground to accurate planeness and covered with a thin layer of insulating material, e.g. tetrafluoroethylene. This type of inductor is complicated and expensive to manufacture, and it is also difficult to produce with good precision, since the spark-machining needs a certain amount of space and, among other things, precludes the cutting of slits of a width less than approx 0.2 mm. The inductors moreover are relatively cumbersome and cannot therefore be used in the welding of small details, especially if the available space is limited. The welding in a detailed pattern is also rendered difficult in that the magnetic field, owing to the relatively large cross-sectional area of the induction coil, will be undefined and not suitable for welding in narrow, linear areas. Finally, the large cross-sectional area of the induction coil entails a further disadvantage from a point of view of energy, since the magnetic field generated will be relatively long, so that the greater part of the magnetic field, that is to say, the non-utilizable part of the magnetic field will be situated inside the inductor, which appreciably diminishes the efficiency of the inductor.
For the above reasons it is generally felt desirable in the packaging industry that an inductor should be provided, suitable for the induction welding of laminated material, especially for the welding of small details in a complicated welding pattern.
It is an object of the present invention to provide an inductor of this type which is not subject to the disadvantages of the earlier known arrangements.
It is a further object of the present invention to provide an inductor whose induction coil can furnish with good precision a desired pattern rich in detail.
It is a further object of the present invention to provide an inductor which furnishes a magnetic field concentrated onto the working surface and which consequently has a high efficiency.
It is a further object of the present invention to provide an inductor which can be manufactured with great accuracy and with slits of a width of less than 0.2 mm.
It is a further object of the present invention to provide an inductor which in an inexpensive manner can be manufactured with the help of known technology and which is particularly suitable for production in large numbers.
These and other objects have been achieved in accordance with the present invention in that a method for the manufacture of an inductor and an inductor for induction welding have been given the characteristic that a base of an electrically insulating material is covered with an electrically conducting material. In a preferred embodiment, the base is covered with an electrically conducting foil which foil is subsequently provided with slits which jointly define an induction coil of the desired shape in the foil.